Cathode assembly for electron tube



Oct. 26, 1965 E. R. LARSON CATHODE ASSEMBLY FOR ELECTRON TUBE Filed Dec. 11, 1961 IN VEN TOR. felvsr K Z/W A/ BY 2 Armin/E) United States Patent 3,214,626 CATHODE ASSEMBLY FOR ELECTRON TUBE Ernst R. Larson, Newark, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 11, 1961, Ser. No. 158,488 9 Claims. (Cl. 313-270) This invention relates to improved cathode assemblies for electron tubes.

One type of presently available electron tubes includes a plurality of concentric electrodes supported on lead-ins and support conductors sealed through a ceramic disk header wafer. In the fabrication of such a tube the electrodes and lead-ins are assembled in a jig in desired strain-free contacting relationship and then brazed to gether into a rigid electrode mount assembly. A cuplike envelope member, which may be of metal or ceramic, is disposed over the electrode mount assembly and sealed to the periphery of the header after tube bake-out and exhaust to form a vacuum-tight envelope. However, the cathode of such a tube is of the indirectly heated type and includes an electron emissive coating thereon. Inasmuch as this emissive coating cannot withstand the high temperatures encountered during the brazing step, the cathode is added after brazing of the rest of the mount assembly .and before the envelope is put in place.

The cathode assembly of such an electron tube comprises a telescoped assembly of two tubular or sleeve parts. A first tubular member serves as the support sleeve and is part of the brazed assembly. A second tubular member closed at one end and providing the cathode is telescoped over the support sleeve after the mount as sembly has been brazed together. This second member is coated with electron emissive material on the external surface thereof. The envelope is then placed over the mount assembly and in contact with the header. During the subsequent heating and exhaust processing the support and cathode sleeve are permanently sintered together. Sealing of the envelope is performed after bakeout and exhaust.

During the evacuation and sealing processing, the tube may be oriented in an inverted position. Because of size variations encountered in mass production of the support and cathode sleeves, the sleeves are not always of such size as to insure tight frictional engagement to maintain them in secure telescoped contact until sintered together. As a result, often times before actual sintering together of the sleeves took place, the cathode sleeve would fall off of its inner support sleeve Where a loose fit existed.

' Welding together of the two sleeves is not suitable because their contact may be too slight to enable a good weld to be made. Furthermore, when sintering does take place, it frequently occurs only at a few spaced spots or along a single line thus resulting in small area contact. Such small area contact not only results in inefficient heat transfer from the support sleeve to the cathode sleeve in normal tube operation, but also affects the electrical resistance in the cathode circuit.

.It has been the practice to make the cathode support sleeve cylindrical and have a lap seam extending along the entire length of the sleeve. The lap seam portion was spot welded to maintain the cylindrical shape. This structure did not prove satisfactory. Seamless tubing was then substituted for the lap seam sleeve support. This type of sleeve support is very accurate with respect to diameter and therefore promotes accurate spacing between the cathode and other elements when the cathode cup is inserted over the sleeve. In order to reduce heat conduction from the cathode, it has been the practice to use tubing having a wall thickness of approximately 3,214,626 Patented Oct. 26, 1965 "ice only .0005 inch. This is a fragile support. Great care is required in positioning the cathode cup on the support sleeve to prevent damage to the sleeve such as deformation, distortion, and misalignment of the sleeve with respect to the other tube elements. Despite careful positioning of the cathode cup over the sleeve damage would frequently occur particularly where a press fit was used, that is, where the inside diameter of the cathode cup is very close to the outside diameter of the support sleeve. Further sintering of the sleeves was not always satisfactory.

It is therefore an object of this invention to provide a new and improved indirectly heated cathode assembly structure which avoids or overcomes the problems of prior art structures as set forth above.

It is another object of this invention to provide a novel and improved two-piece tubular telescoped cathode assembly structure, the parts of which can be manufactured with ordinary mass production tolerances and yet which can be readily telescoped and consistently provide excellent interference fitting contact, substantially uniform from cathode to cathode, andwhich promotes uniformity in the electrical characteristics of the electron discharge devices in which they are used.

Briefly, one cathode assembly structure, made according to this invention, comprises a cathode tubular support sleeve of circular transverse section over which a thicker wall cylindrically shaped tubular cathode sleeve or cup is telescoped. To position the cathode support cup on the support sleeve and to obtain a positive contact press fit, I provide the inside wall of the cathode cup with three or more inwardly directed longitudinally extending contact ridges. Preferably four are used. The four longitudinally extending circumferentially spaced contact ridges engage the external surface of the cathode support sleeve. The inside diameter of the cathode cup at its open end is slightly larger than the cathode support sleeve so that the cup may be readily positioned over the support sleeve and forced down over the support sleeve to provide the positive engagement of the ridges with the sup port sleeve. Upon engagement of the cathode ridges with the cathode support sleeve, the cathode support sleeve flexes inwardly .at the four points of contact permitting the cathode to be moved downwardly into position on the cathode support sleeve without distorting, deforming, or misaligning the cathode support sleeve. The longitudinally extending ridges may extend the entire length of the cathode cup but preferably extend roughly onehalf the length of the cup from the top of the cup.

In the drawings:

FIG. 1 is a longitudinal section of an electron tube incorporating this invention;

.FIG. 2 is a transverse section taken along the line 22 of FIG. 1;

FIG. 3 is an enlarged longitudinal section of the cathode cup and its support sleeve taken along the line 3-3 in FIG. 4 made according to this invention and as incorporated in the electron tube shown in FIG. 1; and

FIG. 4 is a transverse section taken along the line 44 of FIG. 3.

In FIGS. 1 and 2, I show an electron tube 10 incorporating my invention. The tube 10 includes a ceramic disk header 12 having a plurality of bores 14- therethrough. A plurality of electrode support conductors 15 and leadin conductors 16 are sealed in vacuum-tight relation in the bores 14, the walls of which have been metallized.

The electrode mount assembly comprises coaxial cylindrical anode and grid electrodes 26 and 23 and cathode electrode assembly 30, respectively, The anode 26 is mounted on a radially extending flange 32, which is in turn mounted on one lead-in conductor 16 and two support conductors 15. The grid electrode 28 is similarly 3 mounted on a radially extending flange 34 which i in turn mounted on one lead-in conductor 16 and two support conductors 15. The cathode assembly 30 includes a tubular cathode support sleeve 36 mounted on a radially extended flange 38, which is supported on one lead-in conductor 16 and two support conductors 15. The cathode assembly 30 also includes a cup-like tubular, emissive cathode sleeve 40 which is disposed over the support sleeve 36, sleeve 40 being coated with a suitable electron emissive material 41.

A coiled heater 4-4 is disposed in the cathode support sleeve 36 and connects to a pair of lead-in conductors 16 which are sealed through the header 12. As previously stated, all of the above-described elements except the cathode sleeve are all assembled in a jig in loose contacting relationships and brazed together at high temperatures into a strain-free rigid mount assembly. A vacuum-tight envelope is provided by a cup-shaped shell 46 sealed to the periphery of the ceramic disk header 12. The shell 46 includes a pair of longitudinal extending arcuate positioning lugs or tongues 47 and 48 which serve to protect the externally extending conductors 16 and facilitate socketing of the tube.

The presence of the emissive coating 41 on the outer cathode sleeve 40 precludes incorporation of the cathode sleeve in the tube prior to the brazing step. Accordingly, only the support sleeve 36 is included as a part of the brazed assembly. After this brazed assembly is removed from the brazing oven, the cathode sleeve 40 with its emissive coating 41 thereon is telescoped over the support sleeve 36. The envelope is then sealed to the header after bake-out and exhaust as described above.

This sealing step is performed by an oven heating step, at a temperature below the temperature involved in the prior brazing step. The sealing step is performed simultaneously with an exhaust and bake-out of the tube. During the exhaust, bake-out, and sealing of the shell 46 to the ceramic header 12, a sufiiciently high temperature is reached to sinter the support 36 and cathode sleeve 40 together in a secure, permanently bonded assembly.

Referring to FIGS. 3 and 4, the cathode assembly structure 30, made according to this invention, includes emissive cathode sleeve 40 closed at one end so that when the two sleeves 36 and 40 are telescoped together, a bottoming of the support sleeve 36 within the emissive cathode cup 40 will provide automatic longitudinal indexing of the telescoped arrangement facilitating telescoping of the two sleeves 30 and 40. One end of the support sleeve 36 is provided with an inturned rim 52 to eliminate sharp edges and act as a guide for the telescoping action.

The cathode cup 40 which may have a wall thickness of four to five times that of the support sleeve is provided on its inside surface with the circumferentially spaced longitudinally extending ribs or ridges 53 which extend about half way along the length of the cathode cup. The inside diameter of the cup 40 at its lip is made slightly larger than the outside diameter of the sleeve 36 so that the cup may be readily inserted over the upper end of the sleeve 36 to guide the cup along the sleeve until the ridges 53 engage the sleeve 36. When the cathode sleeve or cup is pushed home, the ridges 53 upon contacting the cathode support sleeve cause the sleeve to flex inwardly at the four lines of contact permitting the cathode cup to be moved downward into position on the cathode support sleeve without distorting, deforming or misaligning the cathode support sleeve. A positive fit or pressure contact between the support sleeve and the cathode cup is thus obtained and, after the exhaust operation on the exhaust apparatus, the four contact ridges are positively fused or sintered to the cathode support sleeve providing a good electrical connection and substantially eliminating noise due to loose contact between the support sleeve and cathode cu-p.

When assembled, the portions of the support sleeve Cir 4- between the ri-dges 53 are, it is believed, bowed slightly outwardly, as shown in FIG. 4, whereby these portions act as springs to force the contacting portions of the support sleeve radially outward against the ridges 53. This action results in a good fused joint section. This pressure promotes a good sintering action.

The above construction, made according to this invention, permits variations within manufacturing tolerances of the cathode support sleeve 36 and cathode cup 40 and consistently results in a tight telescoping fiit. The springlike characteristic of the bowedout portions of sleeve 36 results in the support sleeve 36 having a self-adjusting size feature. Thus, in either the extreme case where the cathode sleeve is the maximum and the sleeve support is a minimum, or the extreme case wherein a cathode sleeve is a minimum and the support sleeve 36 is a maximum, nevertheless a good high pressure contact is maintained. The flexibility of the support sleeve accommodates for such size relationship variations. The spring action of the support sleeve insures that a tight pressurized telescoped fit is maintained. Such an accommodation pressure fit is not possible with the prior art assemblies wherein two relatively and undistorted cylinders are telescoped together.

What is claimed is:

1. A cathode assembly comprising an outer tubular cathode member and an inner tubular support member mounted within said outer tubular cathode member in contacting telescoped relationship therewith, said outer tubular cathode member being provided on its inner surface with circumferentially spaced longitudinally extending ridges having a pressure fit engagement with the outer surface of said support member.

2. A cathode assembly comprising an outer cylindrically shaped tubular cathode member and an inner cylindrically shaped tubular support member mounted within said outer tubular cathode member in contacting telescoped relationship therewith, said outer tubular cathode member being provided on the inner surface thereof and circumferentially spaced a plurality of longitudinally extending ridges having a pressure fit engagement with the outer surface of said support member.

3. A cathode assembly comprising a tubular support member of cylindrical cross section and a tubular cathode member having a cylindrical cross section, said tubular cathode member being provided on the inside surface with a plurality of parallel longitudinally extending ridges and telescoped over the tubular support member and having pressurized contact therewith to provide external inwardly directed forces to the outside surface of said tubular support member along said ridges to thereby stress and change said tubular support member from its generally cylindrical cross section.

4. A cathode assembly comprising a first thin wall tubular support member having a generally cylindrical cross section when in an undistorted condition and a second thicker wall tubular cathode member having a cylindrical cross section and provided on its inside surface with a plurality of longitudinally extending ridges, said second tubular member being telescoped over said tubular member and having a pressure fit contact whereby said first tubular member is flexibly stressed and changed from its generally cylindrical cross section.

5. A cathode assembly comprising a thin wall tubular support member of cylindrical shape and a tubular cathode member having a cylindrical cross section, said tubular cathode member having a wall thickness of between four to five times that of said tubular support and being provided on the inside surface with a plurality of parallel longitudinally extending ridges, said cathode member being telescoped over the tubular support member in pressurized contact therewith whereby inwardly directed forces are applied to the outside surface of said tubular support member along said ridges to thereby stress and change said tubular support member from its generally cylindrical cross section.

6. A cathode assembly for an electron tube comprising a thin wall tubular support sleeve of generally cylindrical cross section and a tubular cylindrically shaped electron emissive cathode sleeve having a thicker wall than said support sleeve, said cathode sleeve having on its inside surface a plurality of circumferentially spaced longitudinally extending parallel ridges, said ridegs extending along said cathode sleeve, said emissive cathode sleeve being telescoped with a force fit over said support sleeve, whereby said sleeves are in firm contact with each other in a plurality of substantially uniformly circumferentially spaced longitudinally extending areas registering with said ridges.

7. A cathode assembly for an electron tube comprising a thin wall tubular support sleeve of generally cylindrical cross section and a tubular cylindrically shaped electron emissive cathode sleeve having a wall thickness of four to five times the thickness of said support sleeve, said cathode sleeve having on its inside surface a plurality of longitudinally extending ridges, said emissive cathode sleeve being telescoped over said support sleeve and having a force fit with said support sleeve whereby said sleeves are in firm contact with each other in a plurality of substantially uniformly circumferentially spaced longitudinally extending areas registering with said ridges, and whereby said first tubular supporting sleeve is flexed inwardly at said areas of contact.

8. A cathode assembly comprising a first tubular support member of cylindrical cross section and a tubular cup-shaped cathode member having a cylindrical cross section, said cathode member being provided on the inside surface thereof with a plurality of parallel longitudinally extending ridges extending from the closed end of the cathode toward the open end of said cathode along a portion of the cathode, the open end of said cathode member having a slightly larger diameter than said support member whereby said cathode member may be readily telescoped over the tubular support member and having a pressurized contact therewith to provide external inwardly directed forces to the outside surface of said support member along said ridges.

9. A cathode assembly comprising a first tubular support member of cylindrical cross section and a tubular cup-shaped member having a cylindrical cross section, said tubular cathode member being provided on the inside surface thereof with a plurality of parallel longitudinally extending ridges extending from the closed end of the cathode toward the open end of said cathode along a portion of the cathode, the open end of said cathode having a slightly larger diameter than said support member whereby said cathode member may be readily telescoped over the tubular support member and have a pressurized contact therewith to provide external inwardly directed forces to the outside surface of said support member along said ridges, said support member having a substantially thinner wall than said cathode member.

References Cited by the Examiner UNITED STATES PATENTS 2,244,356 6/41 Bucklin 3l3356 X 2,266,622 12/41 Green 313-356 X 2,310,983 2/43 Miller 313346 X 2,524,001 9/50 Spencer 313337 GEORGE N WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner. 

1. A CATHODE ASSEMBLY COMPRISING AN OUTER TUBULAR CATHODE MEMBER AND AN INNER TUBULAR SUPPORT MEMBER MOUNTED WITHIN SAID OUTER TUBULAR CATHODE MEMBER IN CONTACTING TELESCOPED RELATIONSHIP THEREWITH, SAID OUTER TUBULAR CATHODE MEMBER BEING PROVIDED ON ITS INNER SURFACE WITH CIRCUMFERENTIALLY SPACED LONGITUDINALLY EXTENDING RIDGES HAVING A PRESSURE FIT ENGAGEMENT WITH THE OUTER SURFACE OF SAID SUPPORT MEMBER. 